This problem occurred when viewing the Crawling page and I hit the refresh button on my browser in order to update the progress data displayed.
This problem happened to me twice. The only solution so far has been to completely reinstall the program, with loss of all the data.
To refresh/update the progress data on the Crawling page, I suggest simply clicking on another tab, like the Documentation tab, and then clicking back to the Crawling page. Do not use the refresh button on your browser when a craw is in progress.